This invention relates to improved processes and apparatus for spray drying solids-containing liquids, and more particularly, it relates to improvements in such spray drying apparatus having improved efficiency in the use of energy to produce a final spray dried product.
The use of spray driers to provide solid products, and in particular granular or other particulate solid products, from solids-containing liquids is very well known. Generally, such processes are carried out by heating a drying gas by direct or indirect furnace means and introducing the heated gas into a usually vertical column, called a spray drying tower. The tower is also fed with the solids-containing liquid to be dried, the liquid being suitably introduced into the tower, as through spray nozzles, to break up or atomize the liquid into droplets.
The liquid droplets are thus intimately contacted with the drying gas so that the gas can remove a high percentage of the liquid portion of the solids-containing liquid, or slurry, and thus provide solids containing a greater or lesser quantity of the original solvent or dispersing liquid. The dried solids so obtained are removed from the spray tower continuously or incrementally, and the heated gas, which now contains a quantity of the solvent or dispersing liquid, is removed from the tower. Such spray drying towers can be very effective for producing large quantities of particulate solids with relatively good efficiency due to the direct contact between the drying gas and the liquid.
The particulate solids produced in the tower are usually recovered at a temperature which is substantially below the boiling point of the solvent or dispersing liquid. Conventionally, a large quantity of the drying gas has been vented into the atmosphere, perhaps after some solids and/or entrained liquid separation carried out in a cyclone separator for the purpose of removing the small particles which are frequently unavoidably entrained in the drying gas. More recently, with the emphasis on control of emissions from industrial processes, methods have been suggested for recovering a greater quantity, and preferably all, of the particulate material in the drying gas vented from the tower.
One such method is shown in U.S. Pat. No. 4,171,243. The process illustrated there involves the conventional steps of heating the gas, introducing it into a spray drying tower to dry solids, and removing the gas from the tower. Thereafter, the tower effluent gas is led through a cyclone to remove larger particulate solids, and a quantity of gas is then subjected to further treatment, as in an electrostatic separator, to remove finely divided particulates and other contaminants. In order to reduce the load on the electrostatic precipitator and to reduce the quantity of certain organic contaminants emanating from the spray tower in that process, a portion of the drying gas is recycled to the furnace in an attempt to burn the particulates.
This recycled material thus becomes part of the drying gas. The portion which is recycled is either introduced into the furnace directly or is optionally passed through a dust filter to remove fine particulates before they enter the furnace. Caution must be exercised in introducing material into the furnace, lest it adversely affect the product.